The present invention relates to oxide superconductors and process for production thereof.
As conventional oxide superconductors, generally known are Bi--Sr--Ca--Cu--O superconductors mainly composed of bismuth, strontium, calcium and copper which was found by Maeda, general research worker, and others of National Research Institute for Metals on Jan. 20, 1988, and Bi--Sr--Ca--Mg--Cu--O oxide superconductors mainly composed of bismuth, strontium, calcium, magnesium and copper, Bi--Pb--Sr--Ca--Cu--O oxide superconductors mainly composed of bismuth, lead, strontium, calcium and copper, and Bi--Pb--Sr--Ca--Ba--Cu--O oxide superconductors mainly composed of bismuth, lead, strontium, calcium, barium and copper which are obtained by substitution of elements of the above-mentioned Bi--Sr--Ca--Cu--O oxide superconductors.
However, the above Bi--Sr--Ca--Cu--O oxide superconductors suffer from the problems in that high temperature phase where critical temperature at which electrical resistance becomes zero (hereinafter referred to as "T.sub.C.sup.ZERO ") is about 110 K is formed with difficulty and intermediate temperature phase of T.sub.C.sup.ZERO being about 80 K and heterogeneous phases differing in crystal or composition are apt to be formed. Bi--Sr--Ca--Mg--Cu--O oxide superconductors comprising Bi--Sr--Ca--Cu--O to which Mg is added have a defect in that T.sub.C.sup.ZERO decreases with addition of Mg as shown, for example, in "Japanese Journal of Applied Physics", Vol. 27, No. 12 (December, 1988), pages L2330-L2332.
In case of Bi--Pb--Sr--Ca--Cu--O oxide superconductors which comprise Bi--Sr--Ca--Cu--O to which Pb is added, high temperature phase is easily formed, but they have defects in that range of firing temperature for forming the high temperature phase is narrow and intermediate temperature phase tends to remain. In case of Bi--Pb--Sr--Ca--Ba--Cu--O oxide superconductors which comprise Bi--Pb--Sr--Ca--Cu--O to which Ba is added, T.sub.C.sup.ZERO rises with addition of Ba, but there is a defect in that heterogeneous phases such as BaCuO.sub.2 and BaBiO.sub.3 which are different from the desired high temperature phase are produced.